1. Field of the Invention
The present invention generally relates to multiple-beam scanning devices and image forming apparatuses having the multiple-beam scanning devices. More specifically, the present invention relates to a scanning device used for a writing system of an image forming apparatus such as a digital copier, a laser plotter, a facsimile, or a multiple functional product thereof. Especially, the present invention relates to a multiple-beam scanning device whereby plural optical beams are simultaneously scanned so that an image is written on an image carrier body surface at a high speed, and the image forming apparatus having the multiple-beam scanning device.
2. Description of the Related Art
In tandem type multiple-color image forming apparatus as disclosed in, for example, Japanese Laid-Open Patent Application No. 2002-341273, image carrier bodies such as photosensitive drums corresponding to each of colors such as yellow, magenta, cyan, or black are arranged in a conveyance direction of a transferred body and toner images formed by image forming parts (imaging forming stations) of each of the colors are overlapped. Therefore, a color image can be formed by a single path and high speed image forming can be conducted.
On the other hand, a multiple-beam scanning device has been suggested as a high speed optical scanning device. In the multiple-beam scanning device, since plural beams are scanned in a lump, it is possible to simultaneously record plural neighboring lines and high speed scanning can be conducted without raising the rotational speed of a polygon scanner as an optical deflection part.
However, in the multiple-beam scanning device, a belt area corresponding to the number of beams arranged in the sub-scanning direction is scanned by a single scanning of the polygon scanner. Therefore, if a joint part of a “n”th scanning area and neighboring “n+1”th or “n−1”th scanning area is shifted due to vibration or face tangle of a polygon mirror forming the polygon scanner, arrangement of pixels forming dots in an area corresponding to the joint part and other areas become irregular. Alternatively, the sizes of pixels become uneven so that concentration unevenness or color change may occur.
In addition, generally, the diameter of a beam spot is larger than an interval of scanning lines and a photosensitive body is exposed so that parts of the beam spots are overlapped at the neighboring scanning lines. Therefore, the amount of electrical charge remaining as an electrostatic latent image due to reciprocal law failure of the photosensitive body of a line image formed by a single scanning by a single surface of the polygon mirror is different from that of a line image formed with a certain time by plural scanning even in a neighboring surface. The density of the line image formed with the certain time by plural scanning even in the neighboring surface may be higher.
As a method for avoiding color change or the density unevenness at the joint part in the scanning area of each of surfaces of the polygon mirror standing out, several examples have been suggested. For example, Japanese Patent No. 2685345 discloses an example where the light amount of a light beam is changeable corresponding to the joint part. Japanese Patent No. 2628934 discloses an example where an interval of a light beam is changeable corresponding to the joint part.
In addition, Japanese Laid-Open Patent Application Publication No. 2003-205642 discloses an example where a light source part generating a supplemental beam separately from a recording beam is provided so that the reciprocal law failure is not generated. Furthermore, Japanese Laid-Open Patent Application Publication No. 2005-182139 discloses an example of use of a VCSEL (vertical cavity surface emitting laser) where plural light emitting sources are arranged two-dimensionally.
Recently, acceleration in the multiple-color image forming apparatus has been progressing. Since the multiple-color image forming apparatus is used for simple printing as an on-demand printing system, high quality image and prevention of the density unevenness and color change are required.
Because of this, a memory storing image information larger than the number of lines scanned by a single surface of the polygon mirror forming the polygon scanner is provided in advance, and a pixel of a dot formed by scanning even the neighboring surface is extracted, and a light amount is switched as discussed in Japanese Patent No. 2685345, so that the joint part may not stand out.
However, memory capacity or a processing circuit for determining the pixel of the joint part is necessary so that cost may be raised. In addition, since a large amount of data is processed, time required for this process cannot be ignored from the perspective of improvement of the printing speed. Furthermore, there is no solution to change of a pitch between an end line of scanning on an optional surface generated by an unexpected reason such as vibration of the polygon scanner and a head line of scanning on a neighboring surface.